This invention relates to surface acoustic wave devices and, more particularly, to improved transducers for surface acoustic wave devices.
In recent years, surface acoustic wave (SAW) devices have been employed in an increasing number of practical applications such as filters and delay lines. Surface acoustic wave devices are becoming particularly important in the construction of electronic signal processing equipment becuase they can be constructed on a plane surface using integrated circuit fabrication techniques and they can operate at UHF and VHF frequencies where other types of planar signal processing elements work with limited effectiveness. In a typical surface acoustic wave device, opposing "combs" of interdigitated metal fingers are disposed on a piezoelectric substrate and a surface acoustic wave can be established on the substrate by electrically exciting the fingers. Conversely, an electrical signal can be induced across the fingers by a surface acoustic wave propagating in the piezoelectric substrate material beneath the transducer. The geometrical configuration of the fingers is selected to yield desired device characteristics; for example, the spacing between fingers typically being chosen as a function of the primary wavelength to be propagated in the surface acoustic wave device.
An unfortunate limitation of conventional surface acoustic wave transducers is the fact that when they are made to operate with relatively low losses there is an inherent reflection emitted from the transducer. This "regeneration wave" (known as an "RW" wave) is an inherent consequence of extraction of acoustic surface wave energy and conversion of this energy to electrical signals, and it cannot be directly eliminated since it is inherent in operation of the transducer. The reflection causes spurious signals which are detrimental to most signal processing applications of the surface acoustic wave device. For example, if a wave is reflected from a receiver transducer it will travel back toward the transducer from which it was originally transmitted and then be again reflected from that transducer back toward the receiver transducer. This gives rise to the so-called "triple-transit" reflections (due to the total of three traversals by the twice reflected signal) which are highly undesirable.
It is one of the objects of the present invention to provide a novel transducer for a surface acoustic wave device, the transducer minimizing the undesirable reflections present in prior art devices without substantially sacrificing the efficiency of the device.